1. Field of the Invention
This invention relates to a slit scanning projection device for slit-scanning the information of a flat scanned surface by the use of a deflector and projecting the information on a light receiving surface.
2. Description of the Prior Art
It is well-known that an optical device which scans a flat scanned surface in a slit formed by the use of a deflector such as a rotatable or pivotable mirror and projects the information of the scanned surface upon a light receiving surface is a projection device which can transmit the information of the scanned surface onto the light receiving surface at high speed and which is highly effective in vibration and mechanism. FIG. 1 of the accompanying drawings is a schematic view showing an embodiment of the conventional scanning projection device using a rotatable mirror as the deflector. A flat scanned surface 1 is determined by a slit, not shown, by the use of a rotatable mirror 2. The flat scanned surface is scanned as a slit-like scanned surface 3 having a width l.sub.0 and this slit-like scanned surface 3 is imaged on a light receiving surface 6 by a projection optical system 5. However, when the flat scanned surface is scanned by the use of the rotatable scanning mirror 2, the difficulty is encountered that the length of the light path between the scanned surface 1 and the light receiving surfacce 6 is varied with the rotation of the rotatable scanning mirror 2. That is, if the position of the rotatable scanning mirror when the distance between the rotatable scanning mirror 2 and the scanned surface is shortest (L.sub.O) is defined as the standard position and the distance between the scanning mirror 2 and the scanned surface 1 when the scanning mirror has been rotated by .theta..degree. from the standard position is L.sub.O +.DELTA.L, then the following expression can be obtained: EQU L=(1/cos.theta.-1)L.sub.O
This .DELTA.L is the amount of discrepancy of the length of light path created by the rotation of the scanning mirror and in the usually conceivable form of use wherein .vertline..theta..vertline..ltoreq.25.degree. and L.sub.O =400 mm, .DELTA.L amounts to a maximum 41.4 mm.
As the means for correcting such variation in the length of light path, Japanese Patent Publication No. 9182/1975 discloses providing parallel-movable mirrors before and behind a projection optical system leading from the rotatable mirror to the image plane to correct the variation in the length of light path and moving the projection optical system to eliminate the fluctuation of the conjugate arrangement. Also, U.S. Pat. No. 3,537,373 discloses causing two lens elements within the projection optical system to act synchronously in parallelism to the optic axis to thereby eliminate any variation in the length of light path and any fluctuation of the conjugate arrangement.
However, where a rotatable mirror is used in a copying apparatus, there is another difficulty in addition to the variation in the length of light path. It is that, as viewed from the light receiving surface 6 side, the scanned surface 1 falls down in accordance with the angle of rotation of the rotatable mirror 2. That is, when the mirror image 4 of the slit-like scanned surface 3 by the rotatable mirror 2 is considered, the mirror image 4 is inclined by the angle of rotation .theta. of the mirror 2 with respect to the optic axis of the projection optical system 5. (This condition will hereinafter be referred to as the swing and tilting). The image 7 of this slit-like scanned surface 3 formed on the light receiving surface 6 by the projection optical system 5 is also swung and tilted by .theta.. When the device is used under the conditions that the slit width l.sub.O is 10 mm and the F-number of the projection optical system is 10, the out-of-focus width .DELTA.l' of the slit image 7 on a photosensitive medium 6 will be as great as about 0.2 mm, as shown in FIG. 2 of the accompanying drawings, if .theta.=25.degree., and such out-of focus width is not allowable in practice.